Why isn't there keg beer in Mexico?

Ah but there is, my friend. Not much, but some. In a few big Mexican clubs, mostly. Especially those run by Americans. Both Tecate and Dos Equis are available in kegs, even here in the U.S., if you look hard enough. According to local beer distributors, it's mostly tradition. Until American drinkers hit the scene, Mexican breweries haven't had any call for beer by the keg (the drunken frat party not being a South-of-the-Border mainstay). Bottle-by-bottle is how their customers were used to drinking it. And importing keg beer is a major headache. So there hasn't been a real reason for the Mexicans to retool to vend cerveza by the vat. But just keep pounding down those brews, S.B., and maybe they'll oblige you eventually.